System and method for measuring flow

ABSTRACT

One embodiment of the present invention can comprise a primary flow measurement system, a secondary flow measurement system in fluid communication with the primary flow measurement system and a control coupled to the primary flow measurement system and the secondary flow measurement system. The controller can comprise a processor and a memory accessible by the processor. The processor can execute computer instructions stored on the memory to calculate a flow rate using the primary flow measurement system, in a first mode of operation, and calculate the flow rate using the secondary flow measurement system, in a second mode of operation. The computer instructions can be further executable to switch between the first mode of operation and the second mode of operation based on a predefined parameter.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of, and claims a benefit of priorityunder 35 U.S.C. 120 of the filing date of U.S. patent application Ser.No. 11/012,750 by inventors Stuart A. Tison and Shiliang Lu entitled“System and Method for Measuring Flow” filed on Dec. 15, 2004 now U.S.Pat. No. 7,150,201, the entire contents of which are hereby expresslyincorporated by reference for all purposes.

TECHNICAL FIELD OF THE INVENTION

Embodiments of the present invention relate generally to flow measuringsystems and more particularly to a system and method for measuring aflow utilizing both a primary flow measurement technique and a secondaryflow measurement technique.

BACKGROUND

A number of flow measurement techniques are currently used to calibratemass flow controllers. Primary flow measurement techniques, which derivetheir accuracy from constants of nature or from other primarymeasurements such as mass and time, include gravimetric techniques,constant pressure techniques and constant volume techniques. The firstprimary technique, gravimetric measurement, involves measuring theamount of mass either gained or lost over a time interval. Gravimetrictechniques are generally sufficient for measuring higher rate flowswhere mass loss is significant, but suffer shortcomings in measuringlower rate flows because they typically have insufficient resolution tomeasure mass flow rates on the order of micro-moles per minute.

The next primary technique, the constant pressure technique, uses avariable volume chamber to keep gas pressure constant. The mass flow ismeasured based on gas state equations with the mass flow rate dependingon the change in volume over time. This technique can work well over arange of mass flow rates, but may require an elaborate system forcontrolling the volume of the pressure chamber. Thus, the constantpressure technique suffers limitations in calibrating mass flowcontrollers because constructing a constant pressure chamber (i.e. avariable volume chamber) can require a significant number of movingparts that can cause mechanical complications. As the range of mass flowrates for which the constant pressure chamber is configured increases,the complexity and cost of the constant pressure system will alsoincrease.

The third primary flow measurement technique, the constant volumetechnique, relies on similar state equations as the constant pressuretechnique, but the mass flow rate is dependent on the change inpressure, rather than volume, over time. This technique has becomepopular for calibrating mass flow controllers because of the simplicityof the system (i.e. there are few moving parts). Again, however, theconstant volume technique suffers deficiencies for calibrating mass flowcontrollers because the constant volume technique can typically only beused over a small range of mass flow rates. This limitation existsbecause, if the flow rate is too high for a given chamber volume,pressure changes associated with the high mass flow rate will be tooabrupt to be accurately measured and may quickly exceed the safetylimitations of the chamber. Although larger constant volume chambers canbe constructed, practical considerations of safety, space and costestablish an upper limit on the capability of this flow measurementtechnique.

In general, the disadvantage of primary flow measurement techniques isthat each technique is limited to a particular flow range where thattechnique's uncertainties and design limitations are best suited. Forthis reason, users must typically employ multiple independent flowcalibrations systems utilizing several different primary techniques tocover the range of flows for industrial measurements. Alternatively,users can employ independent secondary techniques, such as sonic nozzles(also known as critical flow venturis or critical flow nozzles), laminarflow meters, ultrasonic flow meters, coriolis flow meters, thermal massflow meters and others. These can be used over a range of flow rates butmust be continually calibrated.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide systems and methods formeasuring flow that substantially reduce or eliminate the disadvantagesassociated with previously developed flow measurement methods andsystems. More particularly, embodiments of the present invention providea system and method for measuring fluid flow across a range of flowrates by combining the features of a primary flow measurement techniquewith those of a secondary flow measurement technique in order to cover awider range of dynamic flows.

One embodiment of the present invention can comprise a primary flowmeasurement system, a secondary flow measurement system in fluidcommunication with the primary flow measurement system and a controllercoupled to the primary flow measurement system and the secondary flowmeasurement system. The controller can comprise a processor and a memoryaccessible by the processor. The processor can execute computerinstructions stored on the memory to calculate a flow rate using theprimary flow measurement system in a first mode of operation, andcalculate the flow rate using the secondary flow measurement system in asecond mode of operation. The computer instructions can be furtherexecutable to switch between the first mode of operation and the secondmode of operation based on a predefined parameter.

Another embodiment of the present invention can comprise a constantvolume chamber, a sonic nozzle in fluid communication with the constantvolume chamber, one or more valves configured to direct a flow of afluid to the constant volume chamber and the sonic nozzle, sensorsconfigured to read one or more parameters of the fluid in the system,and a controller coupled to the sensors configured to receivemeasurements from the sensors. The controller can comprise a processorand a memory accessible by the processor. The processor can executecomputer instructions stored on the memory to calculate the flow rate asfluid accumulates in the constant volume chamber in a first mode ofoperation, calculate the flow rate as the fluid flows through the sonicnozzle in a second mode of operation, and switch between the first modeof operation and the second mode of operation.

Yet another embodiment of the present invention can include a method formeasuring a flow rate comprising for a first mode of operation,calculating the flow rate as fluid accumulates in a constant volumechamber for a first range of flow rates; for a second mode of operation,calculating the flow rate as the fluid flows through the sonic nozzle;and switching between the first mode of operation and the second mode ofoperation.

Embodiments of the present invention, thus, can use primary measurementtechniques to attain accurate measurement capabilities while usingsecondary techniques to extend the flow measurement range of the primarytechnique. Because embodiments of the present invention use multipleflow measurement techniques, the present invention can provide a flowmeasurement system that is self-calibrating and scalable to a wide rangeof flows.

The present invention provides an advantage over previously developedmass flow measurement techniques by combining both primary and secondaryflow measurements techniques. This allows calibration of a mass flowcontroller over a range of flow rates without requiring an independentprimary technique and secondary technique.

Embodiments of present invention provide another advantage by providinga system for calibrating a secondary flow measurement system based onmeasurements from a primary flow measurement system. Because thesecondary flow measurement system (e.g. sonic nozzle system) can berecalibrated in real time based on flow measurements from the primarytechnique (e.g. constant volume system), the secondary flow measurementsystem does not have to be removed from the flow measurement system tobe recalibrated.

Embodiments of the present invention provide yet another advantage overpreviously developed flow measurement techniques because measurementstaken using the secondary flow measurement technique can be compared tomeasurements taken using the primary flow measurement technique. If anerror is detected, this may mean that the secondary flow measurementtechnique must be recalibrated. Thus, embodiments of the presentinvention can detect when recalibration of the secondary flowmeasurement technique is required.

Embodiments of the present invention provide yet another advantage overpreviously developed flow measurement systems and methods by extendingthe range of primary flow measurement techniques through the use of asecondary flow measurement technique.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments of the presentinvention and the advantages thereof, reference is now made to thefollowing description taken in conjunction with accompanying drawings inwhich like reference numerals indicate like features and wherein:

FIG. 1 is a diagrammatic representation of one embodiment of a flowmeasurement system;

FIG. 2 is a diagrammatic representation of one embodiment of a sonicnozzle;

FIG. 3 is a graph illustrating examples of performance characteristicsfor various embodiments of sonic nozzles; and

FIG. 4 is a flow chart illustrating one method of measuring flow ratesaccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of the present invention are illustrated in theFIGURES, like numerals being used to refer to like and correspondingparts of the drawings.

Embodiments of the present invention provide a system and method formeasuring the flow rate of a fluid (e.g., a liquid, a gas, a gas vapormix or other fluid) across a range of flow rates by combining thefeatures of a primary flow measurement technique with those of asecondary flow measurement technique. In one embodiment of the presentinvention, a primary flow measurement system (e.g., a constant pressuresystem, a constant volume system, a gravimetric measurement system orother primary flow measurement system known in the art) can be in fluidcommunication with a secondary flow measurement system (e.g., a sonicnozzle system, a laminar flow meter, an ultrasonic flow meter, acoriolis flow meter, a thermal mass flow meter or other secondary flowmeasurement system known in the art). A controller can receivemeasurements of fluid parameters (e.g., temperature, pressure or otherfluid parameter known in the art) from sensors associated with theprimary flow measurement system and the secondary flow measurementsystem. The controller, in a first mode of operation, can calculate theflow rate of a fluid using the primary flow measurement system and, in asecond mode of operation, can calculate the flow rate of the fluid usingthe secondary flow measurement system. The controller can automaticallyswitch between modes of operation based on one or more predefineparameters. The flow rates calculated by the controller can be used tocalibrate mass flow controllers, mass flow meters and otherinstrumentation.

FIG. 1 is a diagrammatic representation of one embodiment of a flowmeasurement system 10 employing a primary flow measurement system and asecondary flow measurement system. Flow measurement system 10 caninclude a gas source 15 for providing a gas flow to a constant volumesystem 20 and a sonic nozzle system 30 via a system of gas lines 17.Constant volume system 20 can include a constant volume chamber 25 andsensors for reading the pressure (e.g. pressure sensor 26) temperature(e.g. temperature sensor 28) and/or other parameters of the gas insystem 10. Similarly, sonic nozzle system 30 can include a sonic nozzle35 and instrumentation (e.g. pressure sensor 31 and temperature sensor32), which may be the same as or different than that utilized byconstant volume system 20. Sonic nozzle 35 can have a non-adjustablecross-sectional area or an adjustable cross-sectional area. A system ofvalves (e.g., valve 40 and valve 42) can regulate gas flow through flowmeasurement system 10.

A controller 60 can receive temperature, pressure and/or othermeasurements from the sensors and calculate the flow rate of gas throughsystem 10. Controller 60 can determine the mass or volumetric flow rateof a gas through system 10 based on the state of the gas and theconfiguration of the constant volume system or the sonic nozzle system.This analysis can be carried out by microprocessor 62, based on softwareinstructions 66 stored in computer readable memory 67 (e.g. RAM, ROM,magnetic storage device or other computer readable memory known in theart). Additionally, controller 60 can send control signals to valve 40and valve 42 to configure the set of valves to direct the gas flow tosonic nozzle 35 and/or constant volume chamber 25. It should be notedthat while controller 60 is shown as a single controller, thefunctionality of controller 60 can be distributed among multiplecontrollers which may be part of constant volume system 20 and/or sonicnozzle system 30. Moreover, the input and output signals frommicroprocessor 62 can undergo additional intermediate logic and/orconditioning (not shown for the sake of simplicity), such as digital toanalog conversion.

Gas source 15 can provide a flow of gas through gas lines 17 by virtueof the fact that point 90 is at a higher pressure than points 92 or 94.As would be understood by those of ordinary skill in the art, there area variety of methods to provide this pressure differential across system10. When valve 40 and valve 42 are open, the gas will flow towardspoints 92 and 94 to eventually exit system 10. When only valve 42 isclosed, the gas will flow towards point 92 and out system 10. When onlyvalve 40 is closed, the gas will flow towards point 94 (i.e., throughsonic nozzle system 30). When both valve 40 and valve 42 are closed gaswill accumulate in constant volume chamber 25. By changing which valvesare opened or closed, controller 60 can, thus, regulate the flow of gasto constant volume system 20 or sonic nozzle system 30.

When both valve 40 and valve 42 are closed, in the embodiment of FIG. 1,gas will accumulate in constant volume chamber 25 and, consequently, thepressure in constant volume chamber 25 will increase. In this case,controller 60 can use the change of pressure over time (dP/dt) as thegas accumulates in constant volume chamber 25 to calculate the flowrate. Since the mass of a gas in a fixed volume is dependent on the sizeof the volume, the pressure of the gas and the temperature of the gasand since the volume and temperature do not deviate significantly,controller 60 can approximately calculate the mass flow rate usingconstant volume system 20 as follows:

$\begin{matrix}{\frac{\mathbb{d}m}{\mathbb{d}t} = {\frac{\mathbb{d}P}{\mathbb{d}t}\frac{V}{RT}}} & \left\lbrack {{EQ}.\mspace{14mu} 1} \right\rbrack\end{matrix}$

-   -   P=Pressure    -   T=Temp    -   R=Gas Constant    -   t=time    -   V=volume

As would be understood by those of ordinary skill in the art, EQ 1 iseasily extendable to mixed gases and gas vapor mixtures. Additionally, agas compressibility factor can be used as is understood in the art.Moreover, controller 60 can calculate the mass flow rate based on dP/dtaccording to any scheme known in the art. Additionally, controller 60can be further configured to account for any changes in gas temperaturethat may occur using well known thermodynamic equations. The volumetricflow rate can either be calculated in a manner similar to the mass flowrate or can be calculated based on the mass flow rate and density of thegas in system 10.

According to embodiments of the present invention, the primary flowmeasurement technique may be performed as described in ISO 5725-1 or caninclude rate of rise techniques such as those described in U.S. patentapplication Ser. No. 10/887,591, entitled “Method and System for FlowMeasurement and Validation of a Mass Flow Controller,” by Tison et al.,filed Aug. 7, 2004, which is hereby fully incorporated by referenceherein.

When valve 42 is opened (with valve 40 remaining closed in the exampleof FIG. 1), gas can flow through sonic nozzle 35. In this case,controller 60 can calculate the flow rate using sonic nozzle system 30.The mass flow rate through sonic nozzle 35 can be governed by thetemperature (T) of the gas, the gas heat capacity ratio (γ), the gasconstant (R), the gas density (ρ) (which is either known or can becalculated based on the gas constant, upstream pressure and temperature)and the cross sectional area (A) of sonic nozzle 35 such that:

$\begin{matrix}{{{massflowrate} = {\rho\; A\sqrt{\gamma\;{RT}}}}{or}} & \left\lbrack {{EQ}.\mspace{14mu} 2} \right\rbrack \\{{massflowrate} = \frac{{PA}\sqrt{\gamma\;{RT}}}{RT}} & \left\lbrack {{EQ}.\mspace{14mu} 3} \right\rbrack\end{matrix}$

-   -   A=cross sectional area of sonic nozzle    -   R=gas constant    -   T=Temperature    -   P=Pressure upstream of sonic nozzle    -   ρ=gas density (known or equal to P/(RT))    -   γ=Heat capacity ratio

As shown in EQ. 3, the mass flow rate in system 10 is directlyproportional to the pressure upstream of sonic nozzle 35. Consequently,controller 60 can calculate the flow rate based on the pressure upstreamof sonic nozzle 35 or the gas' density. For current sonic nozzles toapproximately adhere to EQ. 2 and EQ. 3, the upstream pressure (P)should be at least two times the downstream pressure (e.g. the pressureat point 92), though the present invention can incorporate new sonicnozzles that require a smaller pressure differential as they aredeveloped. It should be noted that the cross-sectional area of the sonicnozzle can be adjustable.

Thus, according to one embodiment of the present invention, controller60 can, in a first mode of operation, calculate a flow rate as a fluidaccumulates in a constant volume chamber based on the change in pressureover time and, in a second mode of operation, calculate the flow rate asthe fluid passes through the sonic nozzle based on the density of thefluid or the pressure of the fluid upstream of the sonic nozzle.

In the embodiment shown in FIG. 1, both sonic nozzle 35 and constantvolume chamber 25 are upstream of first valve 40. Consequently, whenvalve 40 is closed and valve 42 opened to direct the gas flow to sonicnozzle system 30, the pressure will initially rise in constant volumechamber 25 until constant volume chamber 25 equilibrates. When thisoccurs, a steady pressure (P) will establish in front of (i.e., upstreamof) sonic nozzle 35. Controller 60 can be configured to delaycalculating flow according to the second mode of operation untilconstant volume chamber 25 has had time to equilibrate.

Current constant volume systems can accurately measure flow rates (e.g.within 1%) in a range of 0.01 standard cubic centimeters per minute(0.01 sccm) to 5 standard liters per minute (5 slpm), while currentsonic nozzle systems can accurately measure flow rates in a range of 3slpm to 30 slpm.

Consequently, embodiments of the present invention can employ a constantvolume system 20 and a sonic nozzle system 30 that have overlappingranges of operability. As the flow rate in system 10 approaches theupper limit or constant volume system 20's range of operability (orother arbitrary limit, typically in the overlap range), controller 60can open valve 42, thereby allowing gas to flow through sonic nozzle 35.As an example of switching between using constant volume system 20 andsonic nozzle system 30 for determining the gas flow rate, assumeconstant volume system 20 employs a 10 liter constant volume chamber 25.If controller 60 detects that the flow rate (e.g. from the rate ofpressure change in system 10) has exceeded 4.5 slpm (i.e. a rate ofpressure change greater than 7 psi per minute), controller 60 can openvalve 42 thereby directing the gas flow to sonic nozzle 35. Similarly ifthe detected flow rate drops below a preset value, say 4 slmp,controller 60 can close valve 42, thereby directing gas flow to constantvolume system 20. Thus, embodiments of the present invention can employa smart system able to transparently switch from low flow ratemeasurements using a constant volume to high flow rate measurementsusing a sonic nozzle (i.e., to switch between the first and second modesof operation). The preset thresholds discussed above were given by wayof example only and any predefined parameter and threshold can be used.Moreover, the embodiments of the present invention can incorporateconstant volume systems and sonic nozzle systems having wider ranges ofoperability as they are developed and the ranges given above do notlimit the scope of the present invention.

According to one embodiment of the present invention, controller 60 canuse the overlap range of operability to self-calibrate to account forchanges in system 10, such as changes in the cross-sectional area of thesonic nozzle. Although the cross-sectional area of the sonic nozzletypically remains stable, tungsten, hexafloride and boron trichloridegases, and other toxic gasses associated with semiconductormanufacturing can leave deposits in sonic nozzle 35 causing the throatarea of sonic nozzle 35 to change. If not accounted for, this can leadto inaccuracies in calculating the mass flow rate using on sonic nozzlesystem 30. Embodiments of the present invention can recalibrate tocompensate for such a change without requiring removal and replacementof sonic nozzle 35.

Using the previous example in which the constant volume system and sonicnozzle system have an overlap range from 3 slmp to 5 slmp, a gas flowcan be introduced to system 10 and controller 60 can calculate the massflow rate from constant volume system 20 using EQ 1, above. If the flowrate is within the overlap range (e.g. 3-5 slmp in terms of volumetricflow rate), controller 60 can cause valve 42 to open, thereby directingthe gas flow to sonic nozzle 70. When constant volume chamber 25equilibrates and a steady pressure is established upstream of sonicnozzle 35, controller 60 can calculate the flow rate using sonic nozzlesystem 30. If the flow rate calculated using sonic nozzle system 30 doesnot match the flow rate calculated using constant volume system 20within an acceptable degree of error, this can indicated that the crosssectional area of sonic nozzle 35 has changed. Controller 60 cancalculate a new cross sectional area (A) for sonic nozzle 35 as follows:

$\begin{matrix}{{A = \frac{massflowrate}{\rho\sqrt{\gamma\;{RT}}}}{or}} & \left\lbrack {{EQ}.\mspace{14mu} 4} \right\rbrack \\{A = \frac{{massflowrate}*{RT}}{P\sqrt{\gamma\;{RT}}}} & \left\lbrack {{EQ}.\mspace{14mu} 5} \right\rbrack\end{matrix}$

-   -   A=cross sectional area of sonic nozzle    -   ρ=gas density (known or equal to P/(RT))    -   R=gas constant    -   T=Temperature    -   P=Pressure upstream of sonic nozzle    -   γ=Heat capacity ratio    -   massflowrate=flow rate calculated from constant volume system.

Controller 60 can now use the new value for the cross sectional area ofsonic nozzle 35 when calculating flow rates through sonic nozzle 35. Inthis manner, embodiments of the present invention can use the overlaprange (e.g., 3 slpm to 5 slpm) to calibrate the sonic nozzle on a realtime basis. The operating characteristics of the sonic nozzle allow forone point calibration and extension to higher flows. Recalibration ofsonic nozzle 35 can occur according to a predetermined schedule or on anad hoc basis. It should also be noted that if sonic nozzle 35 isadjustable, the same methodology can be used to determine thecross-sectional area of sonic nozzle 35 for various sized openings.

Embodiments of the present invention can also use the overlap range tocalculate γ (gamma) for a gas using constant volume system 20. Tocalculate γ, a gas flow can be directed to constant volume system 20 andthe flow rate for the gas can be determined as discussed above. If theflow rate is within the overlap range (e.g. 3-5 slpm), controller 60 candirect the gas flow to sonic nozzle system 30. When a steady pressureestablishes upstream of sonic nozzle 35, controller 60 can calculategamma as follows:

$\begin{matrix}{{\gamma = \frac{\left( \frac{massflowrate}{\rho\; A} \right)^{2}}{RT}}{or}} & \left\lbrack {{EQ}.\mspace{14mu} 6} \right\rbrack \\{\gamma = \frac{\left( \frac{{RT}*{massflowrate}}{PA} \right)^{2}}{RT}} & \left\lbrack {{EQ}.\mspace{14mu} 7} \right\rbrack\end{matrix}$

-   -   γ=Heat capacity ratio    -   A=cross sectional area of sonic nozzle    -   ρ=gas density (known or equal to P/(RT))    -   R=gas constant    -   T=Temperature    -   P=Pressure upstream of sonic nozzle    -   massflowrate=flow rate calculated from constant volume system.

In addition to calculating an unknown gamma, embodiments of the presentinvention can calculate gamma for a gas with a known gamma to determineif the controller must recalibrate. Controller 60 can calculate gammafor a gas for which gamma is already known, such as Nitrogen, using EQ.7 or EQ. 8. If the calculated gamma does not match the known gamma, thiscould indicate that the value for “A” being used by controller 60 isincorrect and should be recalculated. Additionally, the ability tocalculate gamma for gas allows the speed of sound for a gas to bedetermined for various temperatures since the speed of sound for a gasis equal to the square root of γRT.

Controller 60 can thus calculate the flow rate through system 10 using aconstant volume system and a sonic nozzle system over a range of flowrates. The flow rate calculated by controller 60 can be used tocalibrate mass flow controllers, as would be understood by those ofordinary skill in the art. If the constant volume system and sonicnozzle system have overlapping ranges of operability, controller 60 canuse a flow rate calculated using the constant volume system to calculatethe cross sectional area of the sonic nozzle, gamma for a particulargas, the speed of sound for a gas and other parameters.

According to one embodiment of the present invention, the system 10 cancalibrated using a particular gas and this information can be extendedto other gases. For example, the cross sectional area of the sonicnozzle can be determined for the system 10 using Nitrogen gas. Thisvalue can then be used in testing other gases in system 10.

Thus, embodiments of the present invention can include a primary fluidflow measurement system (e.g., a constant volume system), a secondaryfluid flow measurement system (e.g., a sonic nozzle system) and acontroller. In a first mode of operation, the controller can beconfigured to use the primary fluid flow measurement system to calculatea flow rate and, in a second mode operation, can be configured tocalculate the flow rate using the secondary flow measurement system. Thecontroller can automatically switch between the first mode of operationand second mode of operation. If the modes of operation have an overlaprange of operability, the controller can calibrate the secondary flowmeasurement system using a flow rate calculated from the primary flowmeasurement system. Calibration data determined for one gas can then beused for other gases.

FIG. 2 is a cutaway view of one example of sonic nozzle 35 that can beemployed in various embodiments of the present invention. Sonic nozzle35 can have an inlet 102 and outlet 104 and a restricted cross sectionalarea or throat 106. Typically, for sonic nozzle 35 to adhere to EQ. 2and EQ 3, the upstream pressure (e.g. inlet side pressure) must be atleast two to three times the downstream pressure (e.g. outlet sidepressure). For many applications, the primary parameter in selecting asonic nozzle is the cross sectional area or throat diameter, which isgenerally selected based upon the desired flow range. FIG. 3 is a chartrepresenting calculations for exemplary embodiments of sonic nozzles.Axis 110 represents the desired flow range at which the sonic nozzlewill operate, axis 112 represent the pressure upstream of sonic nozzle35 and lines 114 represent sonic nozzles having various throatdiameters. In FIG. 3, if an embodiment of the present invention wasdesigned to operate in a pressure range of 10-100 Torr (approximatelyrepresented by area 116), then, by way of example, a sonic nozzle havingdiameters from 0.2 cm to 1 cm could be selected. According to otherembodiments of the present invention a variable orifice sonic nozzle canbe used. Swagelok Company of Cleveland, Ohio, manufactures needle valvesthat can act as variable orifice sonic nozzles.

FIG. 4 is a flow chart illustrating one method of measuring flow ratesaccording to an embodiment of the present invention. The method of FIG.4, in one embodiment of the present invention, can be implemented as aset of computer instructions that are executable by a computer processorand are stored on a computer memory of a controller. According to oneembodiment of the present invention, the controller, at step 202, canconfigure a set of valves to allow a gas to accumulate in a constantvolume chamber. As the gas accumulates in the constant volume chamber,the controller can receive measurements of gas parameters (e.g.,temperature, pressure and or any other gas parameter known in the art)from one or more sensors (step 204) and can calculate the flow rate ofthe gas (step 206) based on the change in gas pressure over time. Thiscan be done, for example according to EQ. 1.

At step 208, the controller can compare the calculated flow rate to athreshold flow rate 210. If the calculated flow rate does not exceed thethreshold flow rate 210, control can return to step 204. If, on theother hand, the calculated flow rate exceeds threshold flow rate 210,the controller, at step 212, can configure the set of valves to allowthe gas to pass through a sonic nozzle. When the valves are configuredto allow gas to pass through the sonic nozzle, gas may still accumulatefor a short while in the constant volume chamber. To account for this,or to otherwise allow the gas to reach an approximately steady state,the controller, at step 214, can delay calculating the flow rate for apredefined delay period.

The controller can receive measurements of various gas parameters (e.g.,temperature, pressure and or any other gas parameter known in the art)(step 216) and, when the delay period has elapsed, can calculate theflow rate of the gas based on a pressure upstream of the sonic nozzle orthe density of the gas upstream of the sonic nozzle according, forexample, to EQ. 2 or EQ. 3 (step 218). The process of FIG. 4 can bearbitrarily repeated (step 220).

In the embodiment of FIG. 4, the controller switches between a firstmode of operation (e.g., calculating the flow rate based on the changein pressure over time) to a second mode of operation (e.g., calculatingthe flow rate based on the pressure or density of the gas upstream ofthe sonic nozzle) based on whether the calculated flow rate exceeds athreshold flow rate. It should be noted however, that the controller canswitch between modes of operation based on any predefined parameter. Forexample, switching between modes of operation can occur based on themagnitude of the change in pressure over time (dP/dt) or otherarbitrarily defined parameter. Additionally, the parameter used todetermine whether to switch from the first mode to the second mode canbe different than the parameter used to determine whether to switch fromthe second mode to the first mode. For example, the controller canswitch from the first mode of operation to the second mode of operationbased on if (dP/dt) exceeds a particular value and can switch from thesecond mode of operation to the first mode of operation if thecalculated flow rate drops below a predefined value.

Although the present invention has been described in detail herein withreference to the illustrative embodiments, it should be understood thatthe description is by way of example only and is not to be construed ina limiting sense. It is to be further understood, therefore, thatnumerous changes in the details of the embodiments of this invention andadditional embodiments of this invention will be apparent to, and may bemade by, persons of ordinary skill in the art having reference to thisdescription. It is contemplated that all such changes and additionalembodiments are within the spirit and true scope of this invention asclaimed below.

1. A system for determining a flow rate of a fluid, comprising: aprimary flow measurement system; a first flow passage to direct fluidfrom a fluid source to the primary flow measurement system; a secondaryflow measurement system in parallel with the primary flow measurementsystem; a second flow passage to direct fluid from the fluid source tothe secondary flow measurement system; a controller coupled to theprimary flow measurement system and the secondary flow measurementsystem, the controller comprising: a processor; a memory accessible bythe processor, the memory storing a set of computer instructionscomprising instructions executable by the processor to: determine a flowrate using the primary flow measurement system; and determine the flowrate using the secondary flow measurement system.
 2. The system of claim1, wherein the set of computer instruction further comprise instructionsexecutable to: switch between a first mode of operation for determiningthe flow rate and a second mode of operation for determining the flowrate; wherein in the first mode of operation the computer instructionsare executable to determine the flow rate using the primary flowmeasurement system and in the second mode of operation to determine theflow rate using the secondary flow measurement system.
 3. The system ofclaim 1, wherein the primary flow measurement system is one of aconstant volume system, a constant pressure system, or a gravimetricflow measurement system.
 4. The system of claim 3, wherein the secondaryflow measurement system is one of a sonic nozzle system, a laminar flowmeter, an ultrasonic flow meter, a coriolis flow meter, or a thermalmass flow meter.
 5. A system of determining a flow rate of a fluidcomprising: a constant volume chamber; a sonic nozzle in parallel withthe constant volume chamber; one or more valves configured to direct aflow of a fluid to the constant volume chamber and the sonic nozzle;sensors configured to read one or more parameters of the fluid in thesystem; and a controller coupled to the sensors configured to receivemeasurements from the sensors, the controller comprising: a processor; amemory accessible by the processor, the memory storing a set of computerinstructions comprising instructions executable by the processor to:determine the flow rate based on the one or more parameters of the fluidas the fluid accumulates in the constant volume chamber; and determinethe flow rate as the fluid flows through the sonic nozzle.
 6. The systemof claim 5, wherein the set of computer instruction further compriseinstructions executable to: switch between a first mode of operation fordetermining the flow rate and a second mode of operation for determiningthe flow rate; wherein in the first mode of operation the computerinstructions are executable to determine the flow rate as the fluidaccumulates in the constant volume chamber and wherein in the secondmode of operation the computer instructions are executable to determinethe flow rate as the fluid flows through the sonic nozzle.
 7. The systemof claim 6, wherein, in the first mode of operation, the flow rate isdetermined based on the change in pressure over time of the fluid. 8.The system of claim 7, wherein, in the first mode of operation, theinstructions are executable to determine the flow rate based on a changein pressure over time as the fluid accumulates in the constant volumechamber approximately according to$\frac{\mathbb{d}m}{\mathbb{d}t} = {\frac{\mathbb{d}P}{\mathbb{d}t}{\frac{V}{RT}.}}$9. The system of claim 6, wherein, in the second mode of operation, theflow rate is determined based on a fluid pressure.
 10. The system ofclaim 6, wherein, in the second mode of operation, the flow rate isapproximately determined according to${massflowrate} = {\frac{{PA}\sqrt{\gamma\;{RT}}}{RT}.}$
 11. The systemof claim 6, wherein, in the second mode of operation, the flow rate isdetermined based on a fluid density.
 12. The system of claim 11,wherein, in the second mode of operation, the flow rate is approximatelydetermined according to${massflowrate} = {\rho\; A{\sqrt{\gamma\;{RT}}.}}$
 13. The system ofclaim 6, wherein the computer instructions further comprise instructionsto delay determining the flow rate as the fluid flows through the sonicnozzle to allow the pressure in the constant volume chamber toequilibrate.
 14. The system of claim 6, wherein the computerinstructions further comprise instructions executable to: determine theflow rate according to the first mode of operation in an overlap rangeof flow rates; and calculate a cross sectional area of the sonic nozzlebased on the flow rate determined according to the first mode ofoperation.
 15. The system of claim 14, wherein the cross sectional areaof the sonic nozzle is approximately calculated according to$A = {\frac{{massflowrate}*{RT}}{P\sqrt{\gamma\;{RT}}}.}$
 16. The systemof claim 14, wherein the cross section area of the sonic nozzle isapproximately calculated according to$A = {\frac{massflowrate}{\rho\sqrt{\gamma\;{RT}}}.}$
 17. The system ofclaim 6, wherein the computer instructions further comprise instructionsexecutable to: determine the flow rate according to the first mode ofoperation in an overlap range; and calculate a heat capacity ratio ofthe fluid based on the flow rate determined according to the first modeof operation.
 18. The system of claim 17, wherein the heat capacityratio is approximately calculated according to$\gamma = {\frac{\left( \frac{{RT}*{massflowrate}}{PA} \right)^{2}}{RT}.}$19. The system of claim 17, wherein the heat capacity ratio isapproximately calculated according to$\gamma = {\frac{\left( \frac{massflowrate}{\rho\; A} \right)^{2}}{RT}.}$20. The system of claim 5 wherein said one or more valves comprise: afirst valve downstream of the constant volume chamber; and a secondvalve downstream of the sonic nozzle.
 21. A method of determining a flowrate of a fluid comprising: supplying fluid to a constant volumechamber; supplying fluid to a sonic nozzle in parallel with the constantvolume chamber; for a first mode of operation, calculating the flow rateas fluid accumulates in the constant volume chamber for a first range offlow rates; and for a second mode of operation, calculating the flowrate as the fluid flows through the sonic nozzle for a second range offlow rates; and switching between the first mode of operation and thesecond mode of operation.
 22. The method of claim 21, wherein, in thefirst mode of operation, the flow rate is calculated based on the changein pressure over time of the fluid.
 23. The method of claim 22, furthercomprising calculating the flow rate based on a change in pressure overtime as the fluid accumulates in the constant volume chamberapproximately according to$\frac{\mathbb{d}m}{\mathbb{d}t} = {\frac{\mathbb{d}P}{\mathbb{d}t}{\frac{V}{RT}.}}$24. The method of claim 21, wherein, in the second mode of operation,the flow rate is calculated based on a fluid pressure.
 25. The method ofclaim 24, wherein, in the second mode of operation, the flow rate isapproximately calculated according to${massflowrate} = {\frac{{PA}\sqrt{\gamma\;{RT}}}{RT}.}$
 26. The methodof claim 21, wherein, in the second mode of operation, the flow rate iscalculated based on a fluid density.
 27. The method of claim 26,wherein, in the second mode of operation, the flow rate is approximatelycalculated according to${massflowrate} = {\rho\; A{\sqrt{\gamma\;{RT}}.}}$
 28. The method ofclaim 21, further comprising delaying calculating the flow rate as thefluid flows through the sonic nozzle to allow the pressure in theconstant volume chamber to equilibrate.
 29. The method of claim 21,further comprising: calculating the flow rate according to the firstmode of operation in an overlap range of flow rates; and calculating across sectional area of the sonic nozzle based on the flow ratecalculated according to the first mode of operation.
 30. The method ofclaim 29, wherein the cross sectional area of the sonic nozzle isapproximately calculated according to$A = {\frac{{massflowrate}*{RT}}{P\sqrt{\gamma\;{RT}}}.}$
 31. The methodof claim 29, wherein the cross sectional area of the sonic nozzle isapproximately calculated according to$A = {\frac{massflowrate}{\rho\sqrt{\gamma\;{RT}}}.}$
 32. The method ofclaim 21, further comprising: calculating the flow rate according to thefirst mode of operation in an overlap range; and calculating a heatcapacity ratio of the fluid based on the flow rate calculated accordingto the first mode of operation.
 33. The method of claim 32, wherein theheat capacity ratio is approximately calculated according to$\gamma = {\frac{\left( \frac{{RT}*{massflowrate}}{P\; A} \right)^{2}}{RT}.}$34. The method of claim 32, wherein the heat capacity ratio isapproximately calculated according to$\gamma = {\frac{\left( \frac{massflowrate}{\rho\; A} \right)^{2}}{RT}.}$35. The method of claim 21, further comprising: controlling one or morevalves to regulate the flow to the constant volume chamber and the sonicnozzle.